$A$ double convex thin lens made of glass of refractive index $1.6$ has radii of curvature $15 \ cm$ each. The focal length of this lens when immersed in a liquid of refractive index $1.63$ is.......$cm$.

The objective glass of a telescope is an achromat of focal length $90\,cm$. The magnitudes of dispersive powers of the two lenses are $0.024$ and $0.036$. The focal lengths of the two lenses are:

An object $2.4 \ m$ in front of a lens forms a sharp image on a film $12 \ cm$ behind the lens. $A$ glass plate $1 \ cm$ thick,of refractive index $1.50$ is interposed between the lens and the film with its plane faces parallel to the film. At what distance (from the lens) should the object be shifted to be in sharp focus on the film (in $m$)?

$A$ hollow lens is made of thin glass and in the shape of a double concave lens. It can be filled with air,water of refractive index $1.33$,or $CS_2$ of refractive index $1.6$. It will act as a diverging lens,if it is

$A$ convex lens of focal length $0.15 \,m$ is made of a material of refractive index $\frac{3}{2}$. When it is placed in a liquid, its focal length is increased by $0.225 \,m$. The refractive index of the liquid is

$A$ bulb is located on a wall. Its image of equal size is to be obtained on a parallel wall with the help of a convex lens. The lens is placed at a distance $d$ ahead of the second wall. The required focal length will be: